>Beer and wine were also supposedly ready in December, though I don't know why?

I'm just a homebrewer, but I may have an (uneducated) answer to that. It takes ales about 3-4 weeks to ferment at 70F (slower in the cold), and lagers 4-8 weeks at 50F. This USDA source shows European spring barley harvests as ending in late September/October, about 8-12 weeks before the winter solstice/Christmas. This PDF from the University of Vermont shows the 2019 hops harvest peaking in late September, which is pretty typical. Sierra Nevada releases their Fresh Hop IPA each December to maximize hop freshness and showcase the year's harvest, for instance.

Figure a few weeks to get your other harvested goods stored before starting your brew and the timeframe lines up perfectly. You can also hold beer longer to let it mellow (Oktoberfest lagers are called Märzens because you brew them in March and let them sit), especially at low temperatures, so mid-to-late December should be the start of a peak beer season that dwindles slowly into the spring, at which point your winter barley is ready for harvest and a new brewing period begins.

EDIT: I forgot those timeframes are with modern commercial yeasts. If you factor in wild or other pre-modern fermentation methods, the fermentation time increases and no fudge factor or waiting period would be required.